Since you cannot reproduce it and I don't have access to other 32-bit machine with linux & php I close this bug.
I'd conclude with: It seems to be gentoo specific bug.
Even though they could not help as of yet. :(

The relevant code is here:
http://lxr.php.net/xref/PHP_TRUNK/Zend/zend_hash.h#ZEND_HANDLE_NUMERIC_EX
It's not the most beautiful thing and it actually relies on undefined behavior, namely that an integer overflow will result in a wrap-around (signed integer overflow has undefined behavior as per the C standard). It relies on getting a negative number to detect the overflow.
However, it explicitly tests for the sign of the result, so it would appear it could never allow a negative number without the string starting with "-". But apparently, that check is optimized away, depending on compiler flags.
Duplicate of bug #51008.
The Debian patch is probably the way to go.